1. Field of the Invention
This invention relates to a method of conveying meltable particulate solids and more particularly, but not be way of limitation, to conveying meltable hydrocarbonaceous solids into processing equipment operated at elevated pressures.
2. Description of the Prior Art
In a process to convert coal into a low ash, low sulfur fuel it is necessary to remove a portion of the inorganic sulfurous material by a solids removal procedure. An economical method for accomplishing this solids removal procedure is through the use of a critical solvent deashing process such as that disclosed in U.S. Pat. Nos. 3,607,716, 3,607,717 and 3,607,718 assigned to the same assignee as the present invention. In that process, an ash and sulfur containing product from a coal liquefaction process is contacted with a select solvent maintained at an elevated temperature near the critical temperature of the solvent and at an elevated pressure. A portion of the mineral ash and other sulfurous material as well as a portion of the undissolved and soluble coal is caused to separate by gravity from the major portion of the soluble coal and solvent. The remaining substantially ash-free soluble coal then is separated from the solvent by increasing the process temperature in a subsequent separation step.
Heretofore, it has been contemplated that the coal liquefaction product introduced as feed material to the critical solvent deashing process would constitute a molten residuum obtained directly from a vacuum still or other fractionation vessel. The use of such a molten material feed creates several problems in the event the critical solvent deashing process is inoperative for mechanical reasons or its operation is not otherwise presently required. The major problem is that the coal liquefaction product is thermally unstable. At the temperature required to maintain the coal liquefaction product in a molten condition, the soluble coal will polymerize and form materials insoluble under the operating conditions of the critical solvent deashing process. Such polymerization results in the loss of this otherwise soluble material as it will be separated and removed with the ash and sulfurous material rather than the soluble coal products during the deashing process. A further problem arises in providing a practical means of storing the large quantities of molten material before deashing.
It would be highly advantageous to be able to operate the critical solvent deashing process with a solid feedstock. Thus, a solid feedstock can be stored and transported more easily than a molten material. Further, it is not as subject to thermal degradation when in the cold, solid form. The coal liquefaction product can be melted readily and introduced into the critical solvent deashing process as additional feed is required. The melting of the coal liquefaction product, however presents another problem. The conventional method of liquefying meltable solids is to batch melt the solids in large tanks. That technique has the disadvantage that to provide a continuous flow of molten material, the tanks must be large in volume. The tanks also must possess a large surface area for heat transer to melt the material due to the low heat transfer properties of viscous liquids such as coal liquefaction products.
The batch melt technique requires a large capital investment in the melting tanks and the pumps required to transport the molten feed into the deashing process apparatus. Further, because of the long retention times at elevated temperatures to effect complete melting of the tank contents, thermal degradation through polymerization can occur when the molten material is held in the large tanks.